Apparatus for coating microspheres with pyrolytic carbon

ABSTRACT

A fluidized bed coater capable of depositing uniform coatings of high-density, oriented pyrolytic carbon having a smooth microstructure on a charge of particles having an initial surface area of 44,000 cm.2. The deposition is accomplished from an atmosphere containing methane and an inert gas at a temperature of about 1,200* C.

ilite States Patent 1151 3,636,923 McCreary et a1. 1451 Jan. 25, 197254] APPARATUS FOR COATING [56] References Cited ggglIEgSN WITH PYROLYTICUNITED STATES PATENTS 3,249,509 5/1966 Blocher, Jr. ..l76/67 [721 Inventrs: William J- Mccreary; Donald Court, 3,382,093 5/1968 Nack 117 100both ofLos Alamo-51M 3,549,847 12/1970 Clark et a]. 219 1049 2,990,2606/1961 Mungen.... ...23/288 [73] Asslgnee. The United States of Americaas represented by the United states Atomic 3,166,614 1/1965 Taylor..264/21 Energy Commlsslon Primary Examiner-Mervin Stein [22] Filed:Mar. 4, 1970 Assistant Examiner-Leo Millstein 1 pp 16507 Attorney-RolandA. Anderson [57 ABSTRACT [52] Cl ii fif jg g A fluidized bed coatercapable of depositing unifonn coatings 51 I Cl 3 loo of high-density,oriented pyrolytic carbon having a smooth E Ki B microstructure on acharge of particles having an initial sur- 1l7/DIG. 6; 118/400, 76, 303;23/288 S PARTICLE DISEWTRA/NME/V 7' ONE 5 face area of 44,000 cm.*. Thedeposition is accomplished from an atmosphere containing methane and aninert gas at a temperature of about l,200 C.

2 Claims, 2 Drawing Figures ,nulo/zso-sza 3 comma zone- Pmmmmsm 3536.923

1 sum 2 OF 2 INVENTOR. William J McCreary BY Donald 8. Court m -wAPPARATUS FOR COATING MICROSPHERES WITH PYROLYTIC CARBON The inventiondescribed herein was made in the course of, or under, a contract withthe U.S. Atomic Energy Commission. It relates generally to gas-solidcontactors and more particularly to an improved fluidized bed coater foruse in depositing pyrolytic carbon coatings on nuclear fuel particles.

Fluidized bed coaters have been found to be extremely useful devices fordepositing desired pyrolytic carbon coatings on nuclear fuel particles.Generally, the coating method consists of suspending the fuel particlesin a heated, vertical column, usually 1 to 5 inches in diameter, by theupward flow of an inert gas such as helium or argon. A thermallydecomposable gaseous hydrocarbon such as methane, acetylene, or any of anumber of hydrocarbons, singlyor in mixtures, is then mixed with theinert gas before it enters the heated column, and the products formed bythe pyrolysis or cracking of the hydrocarbon gas as it rises through thecolumn coat the fuel particles suspended within the column by the flowof gas.

Depending on the amount and type of gaseous hydrocarbon flowing throughthe column, the contact time, the surface area of the fluidized bed, andthe temperature of the column, various forms of pyrolytic carbon can becoated on nuclear fuel particles. It is possible to control ratherclosely whether the deposited pyrolytic carbon will be oriented orisotropic, whether it will have a high or low density and whether itwill have a rough or a smooth microstructure. By the correct choice ofcoating parameters and use of an appropriate fluidized bed coater,desirable properties can be quite literally tailored into the pyrolyticcarbon coatings.

To be useful for coating nuclear fuel particles, however, a fluidizedbed coater must be capable of producing uniform coatings, that is,coatings having a uniform thickness and uniform properties from particleto particle. The reason for this is that it is not now practical toscreen out or otherwise separate those particles in a particular coatingrun which do not conform to the desired criteria.

It has been found that uniform pyrolytic carbon coatings can readily beattained in small fluidized bed coaters such as that disclosed by Bardet a]. in 6 Carbon 603 (1968). Further by appropriate choice of coatingparameters, coatings of widely varying characteristics can be achievedin fluidized bed coaters of this type. A basic problem with thesecoaters, however, is that they are too small. Large quantities of fuelparticles simply cannot be efficiently coated using them. For example,in the fluidized bed coater disclosed by Bard et al. the charge ofuranium dicarbide fuel particles, 104 to 147 microns in size, usuallyhas an initial surface area of about 1,100 cm. and can contain onlyabout 25 grams of uranium.

It is desirable to produce routinely kilogram quantities ofpyrolytic-carbon-coated nuclear fuel particles. Attempts to scale upconventional fluidized bed coaters in size have met with severalproblems. One difficulty is nonuniform movement of particles within thefluidized bed coating zone. An aspect of this problem is distribution ofparticles in dead space at or near the interface of the verticalfluidized bed with the gas distributor when certain gas flow rates areused. Another difficulty is carryover of fuel particles out of theparticle disentrainment zone with certain gas flow rates.

Using scaled-up conventional fluidized bed coaters in attempts toproduce a high-density, oriented pyrolytic carbon coating with a smoothmicrostructure, applicants have found that the coatings generally have anonsmooth microstructure when deposition rates are high enough to givesuitably high densities. This results is attributed to the formation ofsoot particles in the coating zone and the incorporation of suchparticles into the pyrolytic carbon coating. As used within thisapplication, soot means a soft, loose agglomerate of carbon atoms withamorphous structure. At the relatively low temperatures used (about1,115 to l,300 C.), certain higher hydrocarbons formed by the crackingof the hydrocarbon coating gas may also be a part of this soot.

As a result, a scaled-up conventional fluidized bed coater which iscapable of producing uniform coatings of low-density, spongy pyrolyticcarbon, or low-density, oriented pyrolytic carbon, or perhaps evenhigh-density, isotropic pyrolytic carbon, or high-density, orientedpyrolytic carbon with a rough microstructure, is incapable of providinguniform coatings of high-density, oriented pyrolytic carbon with asmooth microstructure. A high-density, oriented pyrolytic carbon of thistype has been found to have utility as a coating on fuel particles usedin nuclear reactors.

As used within this application, high-density, oriented pyrolytic carbonwith a smooth microstructure has the following properties in anas-deposited condition:

a. Density at least 2.0 g./cm." and no more than 2.1 g./cm.

b. High preferred orientation such that Bacon anisotropy factor (BAF) isgreater than 10.0.

c. Microstructure of polished section smooth in appearance when examinedunder polarized light (as contrasted to a rought or grainy appearance)or under dark field illumination (no generalized gray appearancecharacteristics of rough surface under dark field).

it is therefore an object of this invention to provide a gassolidcontactor for fluidizing discrete small diameter particles.

Another object is to provide an improved fluidized bed coater foruniformly coating nuclear fuel particles with pyrolytic carbon.

An additional object is to provide a fluidized bed coater having a novelgas distributor which provides for a more efficient utilization of thegas for fluidizing an coating solid particles, prevents the formation ofdead spaces in which particles are not adequately fluidized, and at thesame time allows a uniform coating of pyrolytic carbon to be efficientlyapplied.

Yet another object is to provide an improved fluidized bed coater inwhich the properties of the pyrolytic carbon coatings deposited can besubstantially varied from run to run.

A further object is to provide a fluidized bed coater for coatingnuclear fuel particles having a particle size range of to 300 microns attemperatures of l,l50 to 1,300C.

A still further object is to provide a fluidized bed coater capable in asingle run of coating near kilogram quantities of nuclear fuel particleswith highdensity, oriented pyrolytic car bon having a smoothmicrostructure.

These and other objects, which will be apparent to those skilled in theart, are accomplished by incorporating into an otherwise conventionalfluidized bed coater (1) a baffle system, and (2) a novel gasdistributor. The baffle system comprises a central baffle and a wallbaffle. The central baffle is centered in the gas exit to the particledisentrainment zone such that gas exiting from this zone must passthrough an annulus having an entrance area small in comparison to thearea of the exit end of the particle disentrainment zone. Likewise, thewall baffle, located along the wall at the exit end of the main coatingchamber reduces the entrance area to the particle disentrainment zonesubstantially below the area of the exit end of the main coatingchamber. The gas distributor comprises a flat apertured faceplatecovered with intersecting conical apertures such that no flat surfaceremains on the faceplate, and means of connecting these conicalapertures to a gas supply.

Applicants have found that by incorporating these novel features into alarge fluidized bed coater of otherwise conventional design, they areable to confine the fluidized bed such that high flow rates of thecoating gas mixture can be used. The reduced contact time of the gas athigh flow rates prevents soot particles from forming in the coating andparticle disentrainment zones and becoming incorporated into thecoating.

The apparatus described herein has been found to be highly suitable fordepositing uniform coatings of high-density, oriented pyrolytic carbonhaving a smooth microstructure on large quantities of nuclear fuelparticles. In single runs, quantities of uranium dicarbide particlescontaining up to 750 grams of uranium can be given uniform coatings ofthis type of pyrolytic carbon. The average coating rate for this type ofcarbon can be as high as 3.6 microns per hour; however, if the averagecoating rate exceeds this value, the microstructure of the carbon beingdeposited is no longer smooth. The coater is also quite suitable fordepositing pyrolytic carbon coatings having substantially differentcharacteristics.

An understanding of this invention will be facilitated by reference tothe following detailed description and the accompanying drawings,wherein:

FIG. 1 is a sectional view of a fluidized bed coater incorporation thenovel baffle system described herein.

FIG. 2 is a sectional isometric view of the novel gas distributordescribed herein.

The major components of the fluidized bed coater shown in FIG. 1 arecomposed of graphite. The hydrocarbon-inert gas mixture enters thecoater through a gas inlet 1 and passes through a gas distributor 2 intothe fluidized bed coating zone 3. Although the gas distributor 2 mayhave a single-jet aperture as shown in FIG. I or it may have multiplejets or even a diffusion barrier, the preferred gas distributor is thatshown in FIG. 2. The main coating chamber 13 is cylindrical andconnected to the gas distributor by a tapered section 14, the taper ofwhich can vary from 30 to 45. The velocity of the gas mixture issufficiently reduced in the particle disentrainment zone 5 to cause mostof the particles being coated to remain below this zone. A tendency atcertain flow rates of a small, random amount of particles to rise above5 is countered by the central baffle 11, held in place by threecentering pins 6 at the em trance to chimney 7. With the central bafflein place, the wall baffle 4 serves as a mechanical barrier to particlesrising along the walls. Although the particles being coated may strikethe surface of the baffles 4 and 11, they do so with insufficientvelocity to harm the coatings being deposited on them. A thermocouplewell is located within the body of the central baffle 11. Through thiswell thermocouples may be lowered into the fluidized bed coating zone todetermine temperatures within various regions of the zone. The chimney 7is capped 9 so that the gases (mostly inert gas and hydrogen) coming outof the fluidized bed coating zone pass out of the coater through gasexit 10. The bed particles are heated to the coating temperature byheater 12 which surrounds the fluidized bed coating zone 3 and the gasdistributor 2. Heater 12 may be any conventional heater, such as aresistance or induction heater, provided it has sufflcient capacity notonly to heat the bed particles to a selected coating temperature butalso to maintain the fluidized bed coating zone 3 at this temperaturewith only minimal gradients throughout the zone during the course of thecoating operation.

A preferred and novel gas distributor is shown in FIG. 2 in the mannerthat it is incorporated into the fluidized bed coater. The gasdistributor comprises a flat apertured disk 15 fitted flush 16 at itsperiphery into the tapered section 14 of the fluidized bed coating zone3 (see FIG. 1). The face plate of disk 15 is covered with a plurality ofintersecting conical apertures, 17, 17' such that no flat surfaceremains on any portion of the faceplate exposed to the fluidized bedcoating zone. Although the included angle of the intersecting cones l7,17 may vary over a rather wide range, e.g., about 45 to 75, an angle of60 has been found to give excellent results. Means 18 are provided forconnecting these conical apertures l7, 17 to a simple gas plenum 19which is connected in turn by passageway 20 to gas inlet 1. Thedistribution of gas flowing through apertures l7, 17 is not known;however, it is reasonable to believe that a slightly higher proportionof the gas passes through aperture 17 than through any other individualaperture 17. The diameter of disk 15 is not critical to the practice ofthis invention, nor is the number of conical apertures l7, H7 in thefaceplate or the size of the passageway 18 connecting the apertures l7,17 to the gas plenum 19, so long as the flow of coating gas throughapertures l7, I7 is maintained at a proper fluidizing velocity. However,a disk having a 1 Arinch diameter exposed to the fluidized bed coatingzone, with nineteen l/ l 6-inch passageways 18 uniformly spaced, andintersecting cones 17, 17 having internal angles of 60 gave excellentresults, when used with a coater having a main coating chamber 13 (seeFIG. 1) 3 inches in diameter and a tapered section 14 having an includedangle of 30.

In carrying out the operation of this invention, the fluidized bedcoating zone is first brought up to a temperature slightly in excess ofthe desired coating temperature. While a flow of an inert gas sufficientto fluidize the particle charge is maintained through the coater, solidparticles, such as uranium dicarbide, are charged into the fluidized bedcoating zone until a charge with an initial surface area of up to about44,000 cm. is attained. The size of individual particles in a charge islimited only by the convenience with which they can be fluidized.However, it has been found that particles in the size range of to 500microns are particularly suited for coating in fluidized bed operations.When the particles in the bed have achieved the desired temperature, thecoating operation is initiated by mixing a gaseous hydrocarbon with theinert gas flowing into the coater. The introduction of the hydrocarbongas into the coater may change the temperature by 2 or 3 percent.Although the coating temperature can in principle vary over a ratherwide range in so long as the cracking or decomposition temperature ofthe hydrocarbon gas used as a source of carbon is exceeded, it has beenfound in practice that pyrolytic carbon coatings having desiredcharacteristics as fuel particle coatings can be deposited quite readilyin the temperature range of 1,] 50 to l,300 C. The properties of thepyrolytic carbon coating deposited depend on the amount and type ofgaseous hydrocarbon flowing through the coating zone, the contact time,the surface area of the fluidized bed, and the temperature.

Although the apparatus of this invention has heretofore been operated atconstant flow rates of the coating gas, the gas flow rate is notcritical to the practice of this invention except as it may ultimatelyaffect the desired properties of the pyrolytic carbon coating beingdeposited. In certain circumstances, it may be preferable to operate thecoater at one gas flow rate until a certain thickness of pyrolyticcarbon has been deposited and then to increase the flow somewhat.Assuming, however, a constant rate of input of hydrocarbon to afluidized bed with a fixed inventory of particles, there will be adecrease in the radial deposition or coating rate during the course of arun.

The manner in which the apparatus of this invention can be used todeposit uniform coatings of high-density, oriented pyrolytic carbon witha smooth microstructure on near kilogram quantities of nuclear fuelparticles is illustrated by the following example:

A fluidized bed coater was constructed as follows: A main coatingchamber 3 inches in diameter and a fluidized bed coating zone 9% inchesin height were provided. The lower tapered section of the fluidized bedcoating zone had an included angle of 30 and an opening l.4 inches indiameter at its lower end. The conical section which made up theparticle disentrainment zone was about 2% inches in height and had anincluded angle of 60. The opening to this section from the main coatingchamber was 2 inches in diameter, thus creating a J-inch lip around theexit from the main coating chamber. The chimney of the coater was 4%inches in diameter. Suspended within it at the exit from the particledisentrainment zone was a 3'r-inch-diameter central baffle. A gasdistributor having an apertured faceplate as hereinbefore described wassealably afiixed to the lower tapered section of the fluidized bedcoating zone.

The fluidized bed coating zone was heated to l,225 C. and a flow ofhelium sufficient to fluidize the bed begun through the coater. A chargeof uranium dicarbide fuel particles which had previously been coatedwith pyrolytic carbon in a conventional fluidized bed coater was thenintroduced into the fluidized bed coating zone. The charge had aninitial surface area of 44,000 cm.'*. The weight of the charge was 705grams, which included 575 grams of uranium. The particles had an averagesize of microns.

After the fluidized bed had been allowed to come to temperature, methanegas was introduced with the helium and the gas mixture adjusted to a 50percent concentration by volume of methane. The total input flow of thegas mixture was then adjusted to 30 l./min. The coater was operated atthe ambient pressure of Los Alamos, N.M., which is about 580 torr. Undercoating conditions, i.e., a 30l./min. flow of a 50/50 gas mixture ofhelium and methane, the temperature within the fluidized bed variedbetween about 1,170 and L1 80 C. depending on the region within thefluidized bed. Under these conditions, 28 microns of high-density,oriented pyrolytic carbon having a smooth microstructure were depositedon the fuel particles at an average rate of 3 microns per hour. Thedensity of this oriented pyrolytic carbon coating was 2.07 g./cm. andthe Bacon anisotropy factor (BAP) was greater than as deposited. Thecoating thus applied was found on analysis to be uniform in boththickness and properties.

it will be apparent to one of reasonable skill in the art that what hasbeen disclosed is a fluidized bed coater having a novel gas distributorand a novel baffle system in which uniform coatings of high-density,oriented pyrolytic carbon having a smooth microstructure can bedeposited on near kilogram quantities of nuclear fuel particles in asingle run. Further, it will be appreciated that the utility of suchcoater is not limited to the deposition of high-density, orientedpyrolytic carbon having a smooth microstructure, but that such coatermay readily be used to deposit pyrolytic carbon coatings having widelyvarying properties. Finally, it will be appreciated that the size of afluidized bed coater of the type disclosed herein is not limited to thatshown by example but can in principle be much larger.

What we claim is:

1. In a fluidized bed coater having, in combination in vertical sequencebeginning at the bottom, a gas inlet, a gas distributor, a fluidized bedcoating chamber, a particle disentrainment chamber, means for uniformlyheating such chambers, a chimney, and a gas outlet, the improvementsconsisting of:

a. a gas distributor having a flat apertured faceplate covered with aplurality of intersecting conical apertures such that no flat surfaceremains on that part of the faceplate exposed to the fluidized bed,means of connecting said intersecting conical apertures to a gas plenum,and means of connecting said plenum to a gas supply; and

b. a baffle system comprising a wall baffle at the exit from the coatingchamber and a central baffle at the exit from the particledisentrainment chamber whereby particles to be coated are kept within auniformly heated coating zone.

2. The device of claim 1 in which the intersecting conical apertures onthe faceplate of the gas distributor have an included angle of 60.

2. The device of claim 1 in which the intersecting conical apertures onthe faceplate of the gas distributor have an included angle of 60*.